A Missing Link in Magnetic Stars

The future of yellow dwarf stars, like our Sun, is determined almost entirely by their mass. The most massive stars, about eight to 12 times heftier than the Sun, can explode as supernovae, leading to the most extreme objects in the universe—neutron stars and black holes. But low-mass stars, a group that includes our Sun, take a quieter path. As they run out of hydrogen to burn and their cores become denser, their outer layers expand and dissipate, transforming them into red giant stars. When their cores reach their densest state, they shed away their outer layers, leaving behind objects known as white dwarfs. A white dwarf is like a cinder from a fire that is faint and gradually cooling.

All of this takes a very long time on a human scale: tens to hundreds of billions of years. (Our universe is 13.8 billion years old, which means no single white dwarf has gone through the full aging process yet.) To us, the Sun's behavior is so stable and predictable as to appear timeless. But the Sun and stars like it live very dynamic lives and follow a path of dramatic transformations before they settle into white dwarf status. In particular, when a Sun-like star orbits in a pair with another star, that companion star can have a dramatic impact on the Sun-like star's evolution.

A Caltech-led research team jointly led by Kareem El-Badry, assistant professor of astronomy; Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science; and Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, has been tracking the late-life behavior of stars by interpreting data that stream in via Caltech's Zwicky Transient Facility (ZTF) at the Palomar Observatory. Every two days, ZTF's wide-field camera fully scans the night sky of the northern hemisphere; by comparing past and present views of the sky, researchers can find variable objects: that is, those undergoing constant, rapid change.

The research team, known as the ZTF Stellar Group, picked up one such transient object in 2022. The object, assigned the name Gaia22ayj, was notable for its rapidly pulsing signal. Originally, the object was classified as a detached double white dwarf binary—two white dwarf stars circling one another. But further data, acquired with the W. M. Keck Observatory on Maunakea in Hawai'i, led astronomers to question that initial assumption. Tony Rodriguez, a graduate student in the ZTF Stellar Group, remembers looking at the data with his colleagues and concluding that the object's light curve—a measure of its light intensity variation over time—made no sense. "Why would the light curve look like that?" Rodriguez recalls wondering.

Motivated by early data acquired by El-Badry, Rodriguez set out to acquire even more data from other telescopes to better understand what was going on with Gaia22ayj. Based on his experience looking at similar systems, Rodriguez knew that they were looking at a white dwarf star orbited by a low-mass star (and not by another white dwarf as was initially proposed), and that the Gaia22ayj system was likely to be highly magnetic. The rapid spin of the white dwarf in this system also reminded him of white dwarf pulsars, which emit regular pulses of electromagnetic radiation when their poles are pointed toward our viewpoint on Earth. But curiously, this star was pulsing every nine minutes, more slowly than the known white dwarf pulsars. The star also appeared to be transferring mass to its companion white dwarf, unlike the known white dwarf pulsars.

Eventually, Rodriguez deduced that Gaia22ayj was the missing link in the previously established lifecycle of a white dwarf pulsar. "We have already seen two infant systems, white dwarf stars in a binary system whose rapid spin builds up a strong magnetic field. And we had seen lots of adult star systems where the white dwarf star was spinning very slowly," Rodriguez explains. "But this was the first star we've seen that is right in the middle of its 'teenage' phase, when it has already established a strong magnetic field and is just beginning to funnel matter from the companion star onto itself. We have never before caught a system in the act of spinning so rapidly but also slowing down dramatically, all while gaining mass from its companion."

Finding a "teenage" white dwarf pulsar is particularly exciting because this phase of a star's life is so brief, Rodriguez explains. How brief? "About 40 million years," he says. But, as he points out, these systems live for billions of years, so their teenage phase is less than 1 percent of their lifetime. In human terms, that would correspond to perhaps several months of our entire lifetime—if only human teenage angst was so brief!

"The data taken at the W. M. Keck Observatory provided firm evidence that this system had a strong magnetic field and was funneling matter onto the white dwarf," Rodriguez says. "Additional data from the unique instruments available at Palomar Observatory showed that this system is, remarkably, slowing down."

The paper, whose authors were led by Rodriguez and include researchers from four different continents, is titled "A Link Between White Dwarf Pulsars and Polars: Multiwavelength Observations of the 9.36-Minute Period Variable Gaia22ayj," and has been published in Publications of the Astronomical Society of the Pacific.

Rodriguez is a National Science Foundation (NSF) Graduate Research Fellow and a Neugebauer Scholar supported by the France A. Córdova Fund. Caltech's ZTF is funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by Caltech's IPAC.